Two types of replacement heart valve prostheses are generally known in the art. A first replacement type comprises totally mechanical heart valves which effect unidirectional blood flow through the use of a device using a mechanical closure. The more common mechanical heart valves comprise pressure responsive, pressure directed movement of a ball in a cage or tilting or caged discs.
Examples of pressure responsive, pressure directed ball movement devices are found in U.S. Pat. Nos. 3,263,239, 3,365,728, 3,466,671, 3,509,582, 3,534,410, and 3,723,996. Earliest valve designs were strictly concerned with providing a one-way valve which could be used as a replacement for natural mitral and aortic valves. The earliest known artificial caged ball prothesis was first successfully used for treatment of cardiac valve disease in 1953. With improvements in valves and medical procedures, caged valve prosthesis rapidly became commonplace in the early 1960's.
A source of historical and background information in Mechanical Valve Prostheses is found in The Fourth Edition of Thoracic and Cardiovascular Surgery, published in 1983 by Appleton-Century-Crofts, a publishing division of Prentice-Hall, inc. The earliest caged ball valves comprised stainless steel outflow orifice and rib cages and silicone rubbber poppers. Such valves experienced a high incidence of thromboembolism associated with the outflow orifices and rib cages. The silicone rubber poppers after a period of use often became grossly deformed with resulting incompetence.
To slow the degeneration of the silicone rubber poppers, cloth and plastic coverings were provided for the metal parts. Such coverings resulted in effects of wear and tissue growth in the coverings. The tissue growth, especially in the coverings over struts of the cages led to a thickening of the struts which slowed or stopped ball movement. Fibrous growth across the orifice of the valve led to severe valvular stenosis.
The use of hollow metal spheres and metal tracks in later models of the caged ball rib valves have overcome some of the original problems, and improvements continue to be made to make caged rib ball valve safer and more efficacious.
However, problems inherent with the geometry of the caged ball valve also leads to physiological problems with the use of the valve as a heart valve replacement prosthesis. The caged rib ball valve comprises three orifices through which blood must flow. The primary orifice is the orifice through which blood passes from the effluent chamber being valved. From the primary orifice the blood passes through a secondary orifice defined by the cage and ball, the size of which is determined by the height of the cage and diameter of the ball. The third orifice is the hollow cylindrical path between the ball and cage and the surrounding influent chamber into which the blood flows from the effluent chamber.
The three orifice pattern in a caged ball valve requires sometimes difficult tradeoffs to be made in design. For example, when the ball is large, the third orifice is relatively smaller leading to third orifice stenosis. When the ball is small, the primary orifice is small and relatively stenotic. Further, if travel of the ball in the cage is restricted, as may be required by physiological free space in either the ascending aorta or left ventricle of a patient, the second orifice size must be reduced with resulting relative stenosis thereat. For these reasons, even in a caged ball valve without physiological or structural complications, use is restricted by the inherent three orifice geometry.
Disc valves have been made in the form of caged disc valves and tilting disc valves. Disc valves are generally preferred over caged ball valves because of the inherent low profile configuration of the disk valve. One of the major problems with disc valves and in particular with caged disc valves, is thrombogenicity. Other problems comprise obstructive characteristics inherent to the basic geometry of caged disc valves and degeneration of the disc occluder and strut fracture. Also hemolysis with disc prostheses is especially common.
An example a tilting disc valve is found in U.S. Pat. No. 4,892,540. Tilting disc valve prostheses have proved to be more satisfactory than the caged disc valves. The tilting disc valve prostheses generally have less hemolysis, lower cross valve gradients, and little wear of carbon pyrolyte discs. However, the tilting disc prostheses have a tendency to clot, and a strict anticoagulant regimen is required. Also movement of the disc in close relation with the sewing ring generally increases chances of interference by contact with adjacent mural endocardium or aortic intima and requires extra care be taken to prevent interference with movement of the disc.
A second replacement type of heart valve prothesis is the "tissue-type" valve which structurally resembles and functions similarly to at least one of the human heart valves. Such valves are most often harvested from pigs or cows and are mounted on a prosthetic stent with an affiliated sewing ring for attachment to the annulus of the valve being replaced. Problems related to the requirement for anticoagulants are usually short term with "tissue-type" valves and failure of such valves is seldom abrupt.
However, such valves are generally slowly rejected from the patient as a foreign body. The rejection is manifested as motion limiting calcification of the leaflets of the "tissue-type" valve and slowly ensuing functional failure. Such failure commonly necessitates replacement within fifteen years of original implantation. Examples of devices which apply to human and other animal "tissue-type" valvular prostheses are found in U.S. Pat. Nos. 3,656,185 and 4,106,129. Two examples of currently manufactured and marketed "tissue-type" valves are the MITROFLOW.TM. Heart Valve by Mitroflow International, Inc., 11220 Voyager Way, Unit 1, Richmond, B.C., Canada V6X 351 and Bovine Pericardial Valve by Sorin Biomedical, S.P.A., 13040 Saluggia (VC), Italy.